Abstract
Glutamate decarboxylase (GAD) is an enzyme that catalyzes the conversion of glutamate to γ-aminobutyric acid (GABA). We previously reported that rice has two distinct isoforms of GAD (OsGAD1 and OsGAD2). The former carries a C-terminal calmodulin-binding domain that is common in dicotyledonous plants, but the latter does not. In order to know regulatory mechanism of these rice GAD activities, we introduced these wild-type GAD genes and mutated genes lacking the region encoding the C-terminal peptide, respectively, into rice calli via agrobacteria. Transformed callus lines were used to determine content of GABA. Unexpectedly, overexpression of mutated OsGAD2 was 100~200 times higher than that of wild-type calli. Plants regenerated showed the same level of accumulation of GABA in all the tissues examined as in calli. Besides, the plants displayed aberrant phenotypes such as pale color, dwarf and curled leaf. These results suggest that C-terminal portion of OsGAD2 plays as an autoinhibitory domain.
